It is well known to perform a quantitative or qualitative analysis of an aqueous liquid by contacting that liquid with an analytical element containing a combination of reagents capable of yielding a detectable product in proportion to the concentration of the predetermined analyte in the liquid. As used herein, this combination of reagents is termed an interactive composition which is capable of chemical reactivity, catalytic activity, or any other form of chemical or physical interaction that can result in the ultimate production of a change in the element that is detectable with suitable procedures and equipment.
One type of particularly useful analytical elements utilizes enzymatic reactions wherein the analyte, upon contact with reagents in the element, reacts with oxygen in the presence of a suitable enzyme to produce peroxide in proportion to the concentration of the analyte. A detectable product is then produced by the reaction of the peroxide in proportion to the concentration of the analyte in the tested liquid. Such useful elements are described, for example, in U.S. Pat. No. 3,992,158 (issued Nov. 16, 1976 to Przybylowicz et al).
Unfortunately, because of the intrinsic instabilities of certain reagents and the need to dry these down in contact with other materials, the reagents contained therein may deteriorate during storage and thus deleteriously affect the accuracy and reliability of the assay. For example, exposure to air and moisture may adversely affect peroxidase which is often included in an element to catalyze the oxidation of interactive compositions by a peroxide.
Peroxidases are used for diverse purposes, including diagnostic determinations of analytes such as glucose, uric acid, cholesterol, etc. In such determinations, excess peroxidase can be added to an element to overcome the effect of enzyme deterioration during storage. However, enzyme immunoassays using a peroxidase-labeled ligand analog have become important for determining an immunologically reactive ligand such as a drug, antigen or other immunologically reactive compound. In such assays, excess peroxidase cannot be added and the adverse effect of its instability is more prominent because of the relatively low concentration of ligand to be determined.
U.S. Pat. No. 4,283,491 (issued Aug. 11, 1981 to Dappen) describes the stabilization of peroxidase in elements with a vinyl copolymer prepared from specific ethylenically unsaturated polymerizable monomers. These elements further contain reagents needed for the determination of particular analytes such as glucose and uric acid. Immunoassays are not described. The copolymer is included in the element carrier materials in an amount of from about 20 to about 50 weight percent. The remainder of the carrier materials can be one or more of a variety of binder materials, e.g. gelatin, hydrophilic celluloses, poly(vinyl alcohol), polysaccharides, etc.
It has been found, however, that peroxidase is not sufficiently stabilized by the materials described in U.S. Pat. No. 4,283,491 when peroxidase is used as a label in a ligand analog. The problem of peroxidase instability is more acute in the determination of a low level ligand in an immunoassay than in an assay of analytes such as glucose or uric acid which are generally found in test liquids in higher concentrations. Therefore, there is a need for a means to stabilize peroxidase-labeled ligand analogs in dry immunoassays.